In fill level sensors operating according to the FMCW method or pulse-transit time method electromagnetic or acoustic waves are emitted in the direction of a feed material surface (in the following also referred to as “filling material surface”). Subsequently a sensor records the echo signals reflected by the feed material, (in the following also referred to as “filling material”), the container internals and the container itself, and from them derives the respective fill level.
Fill level sensors operating according to a transit time method may, for example, comprise sensors which determine the distance to a feed material surface with the use of ultrasound, radar, guided radar, FMCW radar, pulse-burst radar, CW radar or laser. Furthermore, all the sensors that emit a wave and subsequently receive and evaluate the components of said wave that are reflected by a feed, i.e. filling, material surface may be defined as fill level sensors that operate according to a transit time method.
In the use of acoustic or optical waves the signal generated by the fill-level measuring device generally speaking propagates freely in the direction of the feed material surface to be measured. In the case of devices that use radar waves for measuring the feed material surface, both free propagation in the direction of the medium to be measured, and propagation in the interior of a waveguide that guides the radar waves from the fill-level measuring device to the medium can be considered. In the case of devices according to the principle of the guided microwave the high-frequency signals are guided along a waveguide towards the medium.
On the surface of the medium to be measured some of the incoming signals are reflected, and after a corresponding transit time return to the fill-level measuring device. The non-reflected signal components enter the medium and propagate in it, according to the physical characteristics of the medium, in the direction of the container bottom. These signals are reflected by the container bottom and, after passing through the medium and the overlaid atmosphere, return to the fill-level measuring device.
The fill-level measuring device receives the signals reflected on various locations, and from them determines the distance to the feed material according to known methods.
The determined distance to the feed material is provided towards the outside. The provision can be implemented in analog form (4.20 mA interface) or in digital form (field bus).
The fundamental design of radar fill-level sensors is described in detail in the book by Peter Devine “Füllstandsmessung mit Radar. Leitfaden für die Prozessindustrie” (ISBN 3-00-008216-6).
It has been found that fill-level measuring results that are based on radar fill-level sensors are often not accurate.
DE 10 2006 019 191 A1, WO 2010/071564 A1 and WO 2006/103200 A1 describe methods for fill-level measuring that are intended to improve accuracy.